High power phase shifter



July 18, 1961 E. P. WESTBROOK 2,993,140

HIGH POWER PHASE SHIFTER Filed May 15, 1957 2,993,140 HIGH POWER PHASESHIFTER Edwin P. Westbrook, Aubnrndale, Mass., assignor to High VoltageEngineering Corporation, Burlington, Mass, a corporation ofMassachusetts Filed May 13, 1957, Ser. No. 658,688 Claims. (Cl. 3155.42)

This invention relates to microwave devices and in particular to a phaseshifter for microwave devices capable of handling high power. The phaseshifter of the invention may be inserted in any wave guide; and so,while in the following description the invention is described withparticular reference to its use in the radio frequency circuit of amicrowave linear accelerator, the invention is not limited to thisapplication.

In accordance with the invention, a dielectric rod is introducedlongitudinally into the wave guide, and the amount of phase shift isdetermined by the extent to which the rod extends into the guide. In oneembodiment of the invention, the wave guide into which the phase shifteris to be introduced is provided with an elbow, the outer surface ofwhich is provided with a slot through which the rod is introducedlongitudinally into the wave guide. In another embodiment of theinvention, the rod is introduced axially into a circular wave guide intowhich the radio frequency power is introduced through its lateral walland from which the radio frequency power is removed in the same manner.Since the device operates as a phase shifter, it may also be used forthe purpose of adding effective length to a transmission line. Theinvention is not limited to any particular shape of' rod and includesthe use of more than one rod, in which case the rods preferably areparallel to each other.

The invention may best be understood from the following detaileddescription thereof, having reference to the accompanying drawings, inwhich:

FIG. 1 is a diagram illustrating the use of the invention in connectionwith a microwave linear accelerator;

FIG. 2 is a somewhat diagrammatic view in longitudinal central sectionthrough a phase shifter embodying one form of the invention;

FIG. 3 is a side elevation of the device of FIG. 2; and

FIG. 4 is a somewhat diagrammatic view in perspective of anotherembodiment of the invention.

In certain constructions of microwave linear accelerators, high poweroutput is achieved through use of two klystrons which are alternatelypulsed, so that one klystron feeds power into the accelerating waveguide between pulses of the other klystron. Each pulse for each klystronmay have power as high as two megawatts, with the average power beingfour kilowatts for each klystron. In order that the power output of eachklystron may be delivered into the accelerating wave guide asefliciently as possible, the output cavity of each klystron must becoupled to the accelerating waveguide. If there is improper couplingbetween the klystron and the load, the load will not accept all thepower available, and some of the power will be reflected. Referring toFIG. 1 of the drawing, the output from the klystrons 1, 2 is fed into athree-port junction 3 by wave guides 4 and 5 respectively, and power isfed from the three-port junction 3 to the accelerator wave guide 6 bywave guide 7. -When the first klystron 1 is pulsed, it is desired thatthe entire power output be delivered to the accelerator wave guide 6 andthat the power which is reflected by the second klystron 2 through thewave guide 5 arrive back at the three-port junction 3 in proper phasefor optimum power transfer from the first klystron 1 to the aceler-atingwave guide 6. Similarly, when the second klystron 2 is pulsed, it is thereflection of power through the Wave guide 4 from the first klystron 1which must be properly adjusted. There- 2,993,140 Patented July 18, 1961ice fore, in order that the junction between the wave guide 5 and thejunction 3 may present the proper impedance to the power output of thefirst klystron 1, a phase shifter 9 must be introduced in the wave guide5. Similarly, a phase shifter 8 is introduced in the wave guide 4.Alternatively the length of the wave guides 4, 5 from the klystrons 1, 2might be adjusted at the factory to provide the proper phasing; but inthat event each time a klystron was replaced, one might require completea new wave guide, since the new klystron might have differentcharacteristics. Moreover, in general, adjustments must be made afterthe accelerator is installed, so that the use of phase shifters ispreferable. Evidently these phase shifters 8, 9 must be able to handlehigh power, since as noted, each klystron has a peak power output of 2megawatts. The maximum powers involved in microwave applications duringWorld War II were in the vicinity of 1 megawatt and high powers of 2megawatts are a recent development.

The wave guides 4, 5 and 7 are filled with nitrogen at about 30 p.s.i.g.because the power-handling capacity of the guide is increased bypressurization or evacuation, and a pressure seal is easier to make thana vacuum seal. The pressurized region is separated from the evacuatedregions within the accelerating wave guide 6 and within the klystrous 1,2 by quartz windows 10.

Present type of phase shifters include the dielectricslab type in whicha dielectric slab is supported within the wave guide by metal guiderods. Such phase shifters have not been suitable at high power sincepower break down has developed in the vicinity of these guide rods. Upuntil now high-power phase shifters have included the rotary phaseshifter and also the short-slot hybrid with a pair of non-contactingshorts. The cost of such highpower phase shifters is considerable, andone of the objects of the invention is to provide an inexpensive phaseshifter adapted to handle high power. For example, the cost of ashort-slot hybrid phase shifter is approximately $1,800 whereas the costof a high-power phase shifter constructed in accordance with theinvention is approximately $130.

Referring now to FIGS. 2 and 3, a metal tube 11 is soldered or brazed tothe outer circumferential wall of an elbow 12. An aperture is thendrilled in the outer circumferential wall of the elbow 12. The metaltube 11 may conveniently be used as a jig for this purpose, so that theaperture corresponds to the inside diameter of the metal tube 11. Theinner surface adjacent the junction between the metal tube 11 and theelbow 12 is then flared at 13, as by filing with hand tools, so as topresent a well-rounded surface free from sharp points or projectionswithin the elbow 12. Owing to this necessity for flaring, the metal tube11 should have appreciable thickness. A high cylinder or bushing 14,which may be made, for example of Teflon, is slid inside the metal tube11 against a pin 15 which enters through the wall of the metal tube 11.A dielectric rod 16 is introduced into the elbow 12 through the bushing14 so as to lie along the axis of one of the wave guides 17 betweenwhich the elbow 12 is connected. The bushing 14 provides mechanicalsupport for the rod 16 While insulating it from the walls of the elbow12. The bushing 14 should be withdrawn out of the field in the elbow 12.The rod 16 may be made of any dielectric material but fused quartz is tobe preferred as having the best dielectric strength and also as havinglow losses. The rod 16 may have anouter diameter of A of an inch andpreferably is pointed with the point tapering to a diameter of A of aninch over a length of 6 inches.

A rubber hose 18 is fitted over the metal tube 11 and is held by a hoseclamp '19 which is clamped tightly so as to provide a pressure sealcapable of holding the pres- .encies.

may be made;

sure within the wave guide. A rubber cylinder 20 is inserted in theother end of the rubber hose 18 so as to surround the rod 16 snugly. Theinner surface of the.

rubber cylinder 20 is lubricated'with a grease such as Apiezon, and therod 16 is inserted throughthe rubber cylinder 20 and the Teflon bushing14. A second hose clamp 21 surrounds the'rubber hose 18 at the rubbercylinder 20 but is kept loose. Any suitable bellows mechanism may besubstituted for the rubber hose arrangement.

A linear movement of the rod 16 of approximately 1 foot will provide aphase shift of approximately 180 using the dimensions described above.

The smaller the radius of curvature of the elbow 12, the greater themismatch introduced by the phase shifter because the discontinuity atthe quartz rod 16 is more sudden. That is to say, there is a suddenchange from the empty or pressurized guide to the dielectric-containingguide. On the other hand, the larger the-radius of curvature of theelbow 12, the greater the area of the aperture through which the rod 16is introduced, owing to the increased angle at which the rod enters thewave guide, and therefore the harder it becomes to compensate for themismatch introduced by the aperture. The elbow 12 should therefore havea radius of curvature which effects a good compromise between theseopposing tend- The mismatch introduced by the dielectric rod 16 and itsassociated aperturemay be compensated by any conventional means such'asan inductive iris 22, or alternatively a wave guide transformer, atapered wave guide section, or a stub may be used. An inductiveiris ispreferable to a capacitive iris because of its greater power-handlingability.

Alternative constructions include providing two or three small rods inthe elbow for better matching and in order to provide more phase shifterwith less length. Power handling capabilities may be enhanced by usingoptimum geometry, as by selecting the proper radius of curvature for theelbow 12 as outlined above. In conjunction with additional rods, ofcourse, additional apertures are required in the elbow.

In the embodiment of the invention shown in FIGS. 2 and 3, although themismatch introduced by the rod 16 may be compensated, suchcompensation'introduces difficulties. For example, 'the introduction ofan inductive iris 22 intensifies the electric field, and concentrates itso that it is more likely to break down. In an alternative embodiment ofthe invention, shown in FIG. 4, the mismatch introduced by thedielectric rod is com- 'pensated by the position of the short and by theprovision of an appropriate taper at the end of the circular wave guide.In this embodiment a nearly perfect match Moreover, in the elbow form ofthe invention shown in FIGS. 2 and 3, the hollow metal tube '11 must beflared as it approaches the elbow 12 in order to avoid fieldconcentrations.

Such-flaring is not necessary in the embodiment of the invention shownin FIG.

1 4. Moreover, the embodiment shown in 'FIG. 4 per- Referring to FIG. 4,the phase shifter includes a circular wave guide '23 laterally intowhichthe high frequency power is introduced and removed through waveguides 24. The phase shifter of FIG. 4 may be introduced into a straightline wavguide 25 by providing suitable elbows '26. The dielectric rod 16is introduced into introduced by the dielectric rod 16. Except for thefeatures which have just been'described, the operation and 4 functioningof the embodiment of the invention shown in FIG. '4 is the same as thatshown in FIGS. 2 and 3.

Having thus described the principles of the invention together withillustrative embodiments thereof, it is to be understood that althoughspecific terms are employed, they are used in a generic and descriptivesense and not for purposes of-limitation, with-the scope of theinvention being set forth'in the following claims.

I claim:

l. A high-power phase shifter adapted to introduce phase shift into awave guide comprising an elbow adapted to be inserted in said waveguide, an aperture in the outer circumferential wall of said elbow,means for producing adjustable phase shift in said wave guide comprisinga dielectric rod extending through said aperture into said elbowand'thence along the axis ofsaid wave guide, and means for varying theamount of penetration of said rod into said guide for phase shift.

2. Apparatus in accordance with claim 1 wherein an inductive iris isincluded in said elbow in order to compensate the mismatch introducedbysaid rod and said aperture.

3. A high-power phase shifter comprising a circular wave guide shortedat oneend and having a central aperture at the other end, means forproducing adjustable phase shift in said wave guide comprising adielectric rod extending through said aperture axially into saidcircular wave guide, and means for varying the amount of penetration ofsaid rod into said guide for phase shift means for introducing microwavepower laterally into said circular wave guide near one extremity thereofand meansfor extracting microwave power laterally out of said wave guidenear the other extremity thereof.

4. Apparatus in accordance with claim 3 wherein said circular wave guideis tapered at the end having the aperture in order to compensate themismatch introduced by said rod and said aperture.

5. Means for introducing microwave'power into a microwave linearaccelerator comprising in combination: at least two high power microwavepower sources, a junction, a wave guide connecting each of said powersources to said junction, a wave guide connecting said junction to theaccelerating wave guide of said microwave linear accelerator, means forcausing said power sources alternatively to deliver power to saidjunction, and a phase shifter in each of those wave guides which connectsaid power sources to said junction, each of said power shifterscomprising-at least one dielectric rod and means for introducingsaid'rod axially into the wave guide associated therewith, said phaseshifters being adjusted to prevent delivery 'of'power between said powersources.

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